ULTRA-SENSITIVE SPECTROSCOPIC MEASUREMENT OF RADIOCARBON DIOXIDE IN SAMPLES FOR RELEVANT APPLICATIONS

In recent years, the interest for mid infrared radiation has increased for both fundamental research and industrial applications, thanks to the development of continuous-wave room-temperature sources, such as quantum cascade lasers, which fulfill the requests for relatively high power, narrow emission and compact size. The main absorption lines of simple molecules lie in this spectral region and the study of their absorption spectra can give precise information on their concentration. Both these aspects enabled widespread applications of tunable laser absorption spectroscopy for real-time, in-situ and non-invasive gas sensing. Trace gas detection by optical spectroscopy plays a key role in applications that demand quantitative measurements of extremely small amounts of molecular gases. The enormous relevance and the complex interplay between climate change and anthropogenic influence, has focused the attention on mitigation of the greenhouse gases effects, the well-known increase in the average planetary temperature and the increase in atmospheric concentrations of climatically active gases, including CO2. In this context, the scientific and industrial debate focuses on possible methods for measuring and quantifying CO2 emissions from fossil sources, since the identification of markers to control and monitor the reduction of these sources is a necessary step to implement any mitigation politics. In this respect, radiocarbon method represents the most promising approach to validate the estimations provided by single nations or companies. Indeed, radiocarbon is a natural clock, with a lifetime of about 5,700 years, and the measurement of its concentration is an optimal approach to distinguish "young" samples from very old ones, like fossil fuels, completely depleted in radiocarbon. However, ultra-high detection sensitivity is required to quantify radiocarbon due to its extremely low natural abundance being about one part per trillion (10-12) in the biosphere. Thanks to the combination of continuous wave-cavity ring down spectroscopy (CW-CRDS) and strong absorption from fundamental ro-vibrational molecular transitions in the midinfrared, we can overcome the detection limit imposed by the small amount of molecular gases. Nevertheless, state of-the-art conventional CW-CRD spectrometers in the mid IR cannot reach the minimum detectable absorption level required to detect 14CO2, mainly because of the empty-cavity decay rate fluctuations. To overcome this limitation, about ten years ago a novel high-resolution and high-sensitivity spectroscopic technique was proposed: Saturated-Absorption Cavity Ringdown (SCAR) spectroscopy. This technique has shown to improve by more than one order of magnitude the limits of conventional linear-absorption CRDS, thanks to a sample absorption measurement which is independent from the other cavity losses during the same cavity decay event. Indeed, SCAR, benefiting from both CW-CRDS and saturation spectroscopy, overcomes the limits of linear CRD by measuring in each and every single decay event both the "empty" and the "full" cavity contribution. Thanks to the achieved sensitivity, detection of rare molecular species, such as radiocarbon dioxide, was demonstrated. With this work, we describe disruptive applications of this technique, by accurately measuring radiocarbon dioxide concentrations. In particular, these three application areas were targeted: the biogenic fraction in biofuel and bioplastics; parts of concrete walls from nuclear power plants for decommissioning purposes; dating of archaeological samples from Sumer settlements. Specific, customized sample purification processes as well as measurements schemes have been devised.